The present invention relates to a process for the heat bending of sheet materials and also relates to an apparatus for effecting such bending.
More particularly, the invention relates to a method of bending and forming sheet materials into a predetermined shape by the utilisation of vacuum forming and/or molding techniques following preheating of the material in or on an apparatus containing means to enable the said preheating, vaccuum forming and/or moulding techniques.
The invention is particularly adaptable for use in bending a sheet material such as that known under the trademark CORIAN. This is a material which, due to its physical properties, including its stiffness and brittleness when one face is placed under tension and another under compression as a result of bending, will fail.
Sheet materials generally and more particularly so CORIAN, even though they often possess significant elasticity especially when in large lengths or in large sheets do not have a physical property enabling small radius curves to be placed in the material without damage to the material.
There are in existence a number of heat forming processes whereby materials are preheated to enable a subsequent forming process to be carried out on the heated material.
One such process involves preheating of films for subsequent vacuum forming about a mold, whereby the preheated material conforms to the shape of the mold under vacuum conditions. Many variations of such a process have been in existence for some time, however, it has not hitherto previously been known to bend a stiff material such as CORIAN about a mold and under a vacuum following preheating treatment to a critical temperature to thereby enable the provision of curves and bends in the material having radii which are relatively small. It has also not been previously known in this context to use preheating, thence molding of the preheated material such as CORIAN to produce a curve or bend without the use of a vacuum.
It has been previously known to preheat CORIAN in an oven in an attempt to subsequently produce a curve in the material however, such trials have not been successful due to difficulties in handling of the CORIAN after preheating, difficulties in obtaining and maintaining an even heat distribution prior to and during bending of both small and large sheets, and difficulties in establishing an optimum heating time commensurate with the size and thickness of the material. Problems have also been experienced with cracking of the material, crazing of the material surface and reduced impact resistance of the material when adopting prior art forming methods.
As CORIAN is weakened by improper bending conditions due to the shortcommings of the prior art techniques and due to the particular physical nature of CORIAN and like materials it is desirable to provide an improved method and apparatus for the handling of CORIAN during the bending process from the preheating stage to the final bending stage.
Where a small radiused curve is to be placed in a thick sheet material such as CORIAN, bending to achieve this purpose under room temperature conditions would, due to the brittleness of the material under the bending conditions, result in the destruction of the material.
Previously, where it has been desired to form curves in such materials the effect was achieved by tedious machining and gluing of the material or by using the above described oven techniques. The purpose of the curves formed therein is primarily for functional and aesthetic value and to eliminate joins in fascia areas.
The bending of the material is facilitated by heating of the material to a critical temperature so that it achieves sufficient pliability to enable a vacuum forming operation to be performed in conjunction with a forming mold on the heated article. Alternatively, the vacuum may be eliminated in favour of mold forming only.
The critical heating of the article alters the physical and mechanical properties of the material matrix to enable small radius curves to be produced by bending about a mold without a failure in the material.